DVD Demystified

DVD Technology Primer
Figure 3.6
Frequency masking
and hearing
threshold
ear can adjust its sensitivity in order to pick up soft sounds when not overloaded
by loud sounds. This characteristic causes the effect of temporal
masking, in which you are unable to hear soft sounds for up to 200 milliseconds
after a loud sound and for 2 or 3 milliseconds before a loud sound. 9
Perceptual Coding
103
DVD uses three audio data reduction systems: Dolby Digital (AC-3) coding,
MPEG audio coding, and DTS (Coherent Acoustics) coding. All use mathematical
models of human hearing based on sensitivity thresholds, frequency
masking, and temporal masking to remove sounds that you cannot
hear. The resulting information is compressed to about one-third to onetwelfth
the original size with little to no perceptible loss in quality (see
Table 3.1).
Digital audio is sampled by taking snapshots of an analog signal thousands
of times a second. Each sample is a number that represents the
amplitude (strength) of the waveform at that instance in time. Perceptual
9 How can masking work backward in time? The signal presented by the ear to the brain is a
composite built up from stimuli received over a period of about 200 milliseconds. A loud noise
effectively overrides a small portion of the earlier stimuli before it can be accumulated and sent
to the brain.